1. Field of the Invention
The invention generally relates to current sensing applications, and more particularly to current sensing in radio frequency (RF) power amplifier.
2. Prior Art
Unlike current sensing in low speed circuit such as direct current to direct current (DC-DC) switching regulators or low dropout regulator (LDO), etc., the RF cellular power operates at very high-frequency, typically at frequencies above 1 GHz. Any analog circuits with feedback control loop to process high frequency will necessarily use lots of current and therefore decrease the efficiency of the system. Prior art examples the DC average current is sensed through an amplifier.
FIG. 1 depicts a prior art schematic diagram 100 of a DC average current sensed through an amplifier. The supply of the RF power amplifier 120 is not connected directly to the battery. Rather, it is regulated by a voltage regulator 110. With the RF choke inductor 124, the drain current of NMOS 126 is modulated with RF frequency, but the current through the voltage regulator 110 is the average DC current through MOS 126. Therefore, sensing this DC current is relatively simple with the current sense loop 130, where Isense_dc=Ivbat_dc/K, where K is the aspect ratio between the MOS 126 and the sense MOS 136 and Ivbat_dc is the DC current from the battery flowing through the inductor 124. The drawback of this prior solution is that the voltage regulator 110 consumes a large area and therefore also power, which results in overall power amplifier (PA) efficiency degradation.
In other prior art solutions that PA does not have the voltage regulator to control the supply, rather, the supply is connected directly to VBAT. In order to sense the DC current, an off-chip external resistor Rsense (not shown) that is used connected between the RF choke inductor and the battery. The DC current drops across this resistor to create a sense voltage where Vsense=Idc×Rsense. Vsense is then brought back into chip to perform various signal processing controls. The drawback of this kind of solution is that the external resistor has to be very small, e.g., in the order of milliohms. This resistor can be expensive and consumes a large area. Also, when the sensed voltage is brought back into the chip, the accuracy may be compromised.
Therefore, in view of the deficiencies of the prior art, it would be advantageous to provide a solution that overcomes these deficiencies.